1. Field of the Invention
The present invention relates generally to fiber lasers.
2. Description of the Related Art
When pumped with a light signal (e.g., at wavelengths of 800, 980 or 1480 nm), erbium-doped optical fibers have a broad laser transition near .lambda.=1.55 .mu.m which is substantially coincident with an optical-attenuation minimum in silica glass. Because of this coincidence, their high gain, simplicity and compact size and their inherent compatibility with optical fiber transmission lines and semiconductor pump lasers, erbium fiber lasers are particularly suited for a variety of laser applications (e.g., as repeaters and local oscillators) in fiber-based optical systems (e.g., communication systems and radar systems). The fiber laser's suitability for these applications is considerably enhanced if it can generate a single-frequency output signal.
Single frequency erbium-doped fiber lasers have been described in various references. In a first exemplary reference (Zyskind, J. L., et al., "Short Single Frequency Erbium-Doped Fibre Laser, IEE Electronics Letters, Vol. 28, No. 15, July, 1992, pp. 1385-1387), both optical feedback and wavelength selectivity were realized with fiber gratings. The gratings were formed in the fiber with refractive index changes that were induced with short wavelength (e.g., .about.240 nm) radiation. Singlemode operation was obtained by configuring the cavity (between the gratings) short enough to realize mode spacing that was comparable to the bandwidth of the grating. The fiber was pumped through a wavelength-division-multiplexing coupler.
In a second exemplary reference (Ball, G. A., et al., "60 mW 1.5 mm Single-Frequency Low-Noise Fiber Laser MOPA", IEEE Photonics Technology Letters, Vol. 6, No. 2, February, 1994, pp. 192-194), an erbium-doped fiber laser system was described that had a spacing of .about.2.5 cm between a pair of Bragg gratings. These intracore structures provided cavity feedback and mode selectivity. When the laser was pumped at 1480 nm, the polarization of the output signal was found to be elliptical with .about.85% of the power along the elliptical major axis and .about.15% along the elliptical minor axis. This fiber laser system also incorporated an analog noise feedback loop which suppressed intensity noise in the laser output by modifying a drive current of the system's laser pump.
Although the erbium fiber lasers of these references obtain single-frequency operation, they fail to provide structures capable of realizing linearly-polarized output signals. Linearly-polarized light is required for proper operation of many optical-processing devices. For example, intensity modulators (such as a Mach-Zehnder interferometer) typically combine phase-shifted, linearly-polarized signals to obtain an intensity-modulated output signal.
Accordingly, these conventional fiber lasers must generally be supplemented with a polarizer (e.g., a dichroic polarizer or a polarizing beam splitter) to be compatible with many optical systems. The size and cost of these optical systems are therefore increased. In addition, system efficiency is degraded because a polarizer will significantly reduce the laser's output power.